Topology discovery in optical WDM networks

ABSTRACT

In an optical WDM network, each optical channel is modulated with a respective channel identity. Detectors, conveniently at multiplex ports of optical band filters, detect the channel identities of all of the optical channels in an optical signal at the respective points to produce respective channel lists. A network management system determines channel lists for through ports of the optical band filters, identifies matching pairs of channel lists to determine a topology of each node and to identify optical paths entering or leaving each node, and identifies matching pairs of channel lists for these paths to determine an inter-node topology of the network. The channel identity detector points can alternatively be at the optical paths entering or leaving each node.

[0001] This invention relates to topology discovery in optical WDM(wavelength division multiplex) communications networks or systems.

BACKGROUND

[0002] As communications networks become more complex, the task ofnetwork management becomes increasingly difficult. An important aspectof a network management system, or NMS, relates to determining andmaintaining an accurate record of the topology or connectivity of thenetwork. In optical WDM communications networks this may involveknowledge of not only connections of optical fibers among nodes of thenetwork, but also connections of optical fibers within the nodes,allocation of wavelengths to respective optical fibers, and arrangementsand sequences of multiplexers and demultiplexers, or optical bandfilters, within the nodes.

[0003] While it is conceivable to provide and maintain manually a recordof the topology of a network, this becomes increasingly impractical asthe network becomes more complex, and has other disadvantages such asbeing subject to errors and being slow and inconvenient to update andrespond to changes.

[0004] In communications networks it has been proposed to provideautomatic discovery of the topology of the network. For example, suchproposals are disclosed for ATM networks in Suzuki U.S. Pat. No.5,796,736 issued Aug. 18, 1998 entitled “ATM Network Topology AutoDiscovery Method” and in Chatwani et al. U.S. Pat. No. 5,729,685 issuedMar. 17, 1998 and entitled “Apparatus For Determining The Topology Of AnATM Network Or The Like Via Communication Of Topology InformationBetween A Central Manager And Switches In The Network Over A VirtualService Path”.

[0005] As disclosed in the latter patent, ATM cells include so-calledlink advertisement messages (LAMs) each of which identifies anoriginating switch and port number and is forwarded by a receiving orneighbour ATM switch to a network manager. The network manager therebydevelops information profiling the topology of the network. Matching ofLAM pairs is carried out by the network manager to confirm bidirectionalNNI (Network-Network Interface) links.

[0006] While such a known arrangement may be effective for discoveringneighbours in an ATM network, it requires handling of the LAMsspecifically in each ATM switch and reduces bandwidth of the network fordata traffic. In addition, such a known arrangement is not effective fordetermining the topology of an optical communications network, in whichfor example an optical fiber path between two nodes A and B may passthrough an intermediate node C. The ATM cell or packet level which wouldbe determined by such a known arrangement would indicate that the nodesA and B are coupled together and would not show the node C, whereas theactual topology in this case is that the node A is coupled to the nodeC, and the node C is coupled to the node B.

[0007] Also, Wood U.S. Pat. No. 6,108,702 issued Aug. 22, 2000 andentitled “Method And Apparatus For Determining Accurate TopologyFeatures of A Network” discloses a system for monitoring packet trafficin a network to determine topology features using logical groupings ofports and/or devices. Orr et al. U.S. Pat. No. 5,727,157 issued Mar. 10,1998 and entitled “Apparatus And Method For Determining A ComputerNetwork Topology” discloses determining the topology of a computernetwork including data-relay devices based on a comparison of sourceaddresses heard by the various data-relay devices.

[0008] In an optical network, Fee U.S. Pat. No. 6,108,113 issued Aug.22, 2000 and entitled “Method And System For Transporting AncillaryNetwork Data” discloses superimposing a sub-carrier modulation signal ata relatively low frequency (e.g. 1 MHz), containing ancillary networkdata, on an optical carrier of a high bit rate (e.g. 1 to 10 GHz) datasignal. This patent discloses that the ancillary network data caninclude any of numerous data types identifying any of numerous networkelements, and can be used for any of numerous network managementpurposes one of which is listed as “Probing Network Topology”, but nofurther information in these respects is disclosed.

[0009] Fatehi et al. U.S. Pat. No. 5,892,606 issued Apr. 6, 1999 andentitled “Maintenance Of Optical Networks” discloses an apparatus foradding a dither signal to an optical carrier modulated with aninformation signal, to provide a method for monitoring and trackingend-to-end signal routing in multi-wavelength optical networks. Thispatent discloses that the monitoring can take place at any point in thenetwork. This patent is not concerned with topology discovery.

[0010] A need exists for an improved method of topology discovery whichis particularly applicable to optical WDM networks.

SUMMARY OF THE INVENTION

[0011] According to one aspect of this invention there is provided amethod of determining topology of an optical WDM (wavelength divisionmultiplex) network in which optical signals comprising a plurality ofWDM optical channels are communicated, comprising the steps of:modulating each optical channel with a respective signal comprising achannel identity; detecting the channel identities of all of the opticalchannels in an optical signal at each of a plurality of points in thenetwork to produce a channel list for each of said points; andidentifying matched pairs of channel lists to determine optical paths ofthe network between pairs of said points.

[0012] Another aspect of this invention provides a method of determiningtopology of an optical WDM (wavelength division multiplex) network inwhich optical signals comprising a plurality of WDM optical channels arecommunicated among a plurality of nodes of the network, comprising thesteps of: modulating each optical channel with a respective signalcomprising a channel identity; for each of a plurality of optical pathsentering or leaving each of a plurality of nodes, determining a channellist of all the optical channels in an optical signal on the opticalpath, this step comprising detecting the channel identities of all ofthe optical channels in an optical signal at each of a plurality ofpoints; and identifying matching channel lists to determine opticalpaths of the network between the nodes.

[0013] The step of determining a channel list of all the opticalchannels in an optical signal on an optical path entering or leaving anode can comprise detecting the channel identities of all of the opticalchannels in an optical signal on the respective optical path.

[0014] Alternatively, this step can comprise, for each node; detectingthe channel identities of all of the optical channels in an opticalsignal at a multiplex port of each of a plurality of optical bandfilters of the node to produce a respective channel list M; determininga respective channel list T for an optical signal at a through port ofthe respective optical band filter, the channel list T comprisingchannels of the respective list M which are not within a pass band ofthe optical band filter; identifying matching channel lists M and T todetermine optical paths within the node; and identifying unmatchedchannel lists M or T as channel lists for optical path entering orleaving the node.

[0015] In this case the step of identifying matching channel lists M andT to determine optical paths within the node can comprise identifyingany optically transparent optical band filters of the node for each ofwhich the channel lists M and T are the same; identifying matched pairsof the other channel lists M and T for the node to determine opticalpaths between respective ports of different optical band filters withinthe node; and identifying any channel lists, from among said matchedpairs of channel lists for the node, matching said same channel lists Mand T to determine optical connections of said optically transparentoptical band filters within the node.

[0016] The invention also provides an optical WDM (wavelength divisionmultiplex) network comprising a plurality of nodes and optical paths forcommunicating optical signals, comprising a plurality of WDM opticalchannels, within and among the nodes, the network comprising: a sourcefor each optical channel; a modulator for modulating each opticalchannel with a respective signal comprising a channel identity; aplurality of optical filters for combining optical channels to produceoptical signals and for separating optical signals to derive opticalchannels from the optical signals; a plurality of detectors fordetecting the channel identities of all of the optical channels in anoptical signal at each of a plurality of points in the network toproduce a channel list for each of said points; and a network managementsystem for identifying matched pairs of said channel lists to determineoptical paths of the network between pairs of said points.

[0017] In such a network the optical filters can comprise optical bandfilters each having a multiplex port, an add or drop port, and a throughport, and said plurality of points in the network can comprise multiplexports of the optical band filters. Preferably the network managementsystem is arranged to determine a channel list for a through port of anoptical band filter by omitting, from optical channels of a channel listfor the multiplex port of the respective optical band filter, opticalchannels within a pass band of the optical band filter.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The invention will be further understood from the followingdescription by way of example with reference to the accompanyingdrawings, in which:

[0019]FIG. 1 illustrates a plurality of nodes, a network managementsystem, and communication paths therebetween forming part of an opticalWDM communications network incorporating an embodiment of the invention;

[0020]FIG. 2 illustrates apparatus provided in one or more of the nodesin accordance with an embodiment of the invention;

[0021]FIG. 3 illustrates one form of modulator which can be used in theapparatus of FIG. 2;

[0022]FIG. 4 illustrates one form of detector which can be used in theapparatus of FIG. 2;

[0023]FIG. 5 is a flow chart providing an outline of steps carried outin performing a method in accordance with an embodiment of theinvention;

[0024]FIG. 6 is a flow chart illustrating in more detail steps carriedout for each node in performing the method represented by FIG. 5;

[0025]FIG. 7 illustrates a plurality of optical band filters withreference to which an aspect of an embodiment of the invention isexplained; and

[0026]FIG. 8 is a flow chart illustrating steps carried out for thenetwork in performing the method represented by FIG. 5.

DETAILED DESCRIPTION

[0027] Referring to FIG. 1, a simple optical communications network isillustrated as comprising a plurality of, in this example three, nodes10, identified individually as Node A, Node B, and Node C, which arecoupled together via various optical communications paths 12 representedby solid lines. The paths 12 can comprise optical fibers and/orwavelengths on optical fibers. The network also includes a networkmanagement system (NMS) or station 14, to which all of the nodes 10 arecoupled via communications paths 16 represented by dashed lines. Thepaths 16 can be optical or other (e.g. electrical) communications paths.

[0028] As is known in the art, such a communications network can includean arbitrary number of nodes 10 and NMSs 14, which may be locatedtogether or separately from one another, with various arrangements ofcommunications paths among the nodes 10 and NMSs 14 having any desiredconfiguration or topology. Furthermore, such a communications networkcan be coupled to other similar or different communications networks invarious manners. Accordingly, FIG. 1 serves merely to illustrate asimple form of network for the purposes of describing an embodiment ofthe invention.

[0029] As illustrated in FIG. 1, one of the paths 12, from Node A toNode B, is divided into two parts 12-1 and 12-2 which pass through NodeC as illustrated by a dotted link in Node C. Although this path may passvia one or more optical filters which can drop or add optical channelsin the Node C, it is assumed that no optical channels are dropped fromor added to this path in Node C. Thus at a communications level, thispath only communicates signals from Mode A to Node B. However, thetopology of the network is that this path comprises the path 12-1 fromNode A to Node C, and the path 12-2 from Node C to Node B. Discovery ofthe topology of the network by the NMS 14 is required, for example, toidentify the paths 12-1 and 12-2 via Node C.

[0030] It can be appreciated that, without any interception of signalson the dotted link path at Node C in the network of FIG. 1, an analysisof signals at the communications level will not identify the separatepaths 12-1 and 12-2 via Node C, but merely a signal path from Node A toNode B. It can also be appreciated that this is a very simple example,and that in practice an optical network may include far more than threenodes, which may include multiple optical band filters for multiplexingand demultiplexing numerous different wavelength optical signals in aWDM network, and that there may be far more optical communication paths,so that automatically and accurately determining the topology of thenetwork by the NMS 14 can be a very difficult task.

[0031] In embodiments of this invention, carrying out this task isfacilitated by providing each optical signal within the network with arespective identifier, referred to as a channel identity or CID. The CIDis applied as a relatively low frequency (e.g. about 1 MHz or less)amplitude modulation of the optical signal, referred to as a dithertone, using any desired coding scheme for the CID.

[0032] As one example, each CID may be constituted by a repeatedsequence of 14 bytes of data identifying the CID. Numerous other ways ofencoding unique CIDS for the respective optical signals exist and any ofthese can be used, with appropriate forms of detection and decoding ofthe CIDs.

[0033]FIG. 2 illustrates apparatus provided in one or more of the nodesin accordance with an embodiment of the invention. As illustrated inFIG. 2, each optical signal or channel, for example supplied from arespective one of a plurality of optical sources 20 modulated withrespective data signals (not shown), is supplied via a respective one ofa plurality of electronic variable optical attenuators (EVOAs) 22supplied with respective CIDs, whereby each optical signal or channel ismodulated with a respective CID as indicated above. As illustrated inFIG. 2 for a WDM network, optical signals with their respective CIDs aremultiplexed in an optical multiplexer 24 to produce optical signalswithin an optical band on an optical path 26.

[0034] The optical path 26 is coupled to an add port AD of an opticalband filter (OBF) 28 which serves to multiplex the optical signals onthe optical path 26 with other optical channels supplied via an opticalpath 30 to a through port TH of the OBF 28, These other optical channelson the optical path 30 similarly are provided, for example in the samemanner as the signals from the optical sources 20, each with arespective CID. The multiplexed optical signal produced by the OBF 28 issupplied via a CID detector 32 to an optical path 34.

[0035] The CID detector 32, for example as further described below,detects the CIDs of all of the optical channels present in the opticalsignal at the multiplexed output port of the OBF 28 and hence on theoptical path 34, and supplies these via one of the paths 16 to the NMS14.

[0036] The optical path 34 should lead to, i.e. supply its opticalsignal to, an optical input elsewhere in the optical network. Forexample, it may lead to a through input port of another OBF within thesame node as the OBF 28, or to a multiplex input port of another OBF,acting as a demultiplexer or drop filter, within the same node as theOBF 28, or to an input port of another OBF in a different node from thatcontaining the OBF 28.

[0037]FIG. 2 illustrates the optical path 34 as being coupled via a path36, represented by a dashed line, to an optical path 38, which iscoupled via another CID detector 40 to the multiplex input port ofanother OBF 42 having through and drop output ports TH and DRrespectively providing demultiplexed optical output signals. Like theOBF 32, the CID detector 40 detects the CIDs of all of the opticalchannels present in the optical signal at the multiplexed input port ofthe OBF 42, and hence on the optical path 38, and supplies these via oneof the paths 16 to the NMS 14.

[0038] It can be appreciated from the above description that the opticalpath 36 may comprise an optical fiber between two different nodes, oneof which includes the OBF 28 and the other of which includes the OBF 42,or it may comprise an optical path within a node, the OBFs 28 and 42both being within this one node. As described above, the NMS 14 may beseparate from the nodes, incorporated within a node, or distributedamong nodes, with the paths 16 being provided accordingly.

[0039]FIG. 3 illustrates one form of modulator which can be used in theapparatus of FIG. 2, for implementing each of the EVOAs 22 as shown inFIG. 2.

[0040] Referring to FIG. 3, an optical signal to be provided with a CIDis supplied via an optical path 50, an EVOA 52, and an optical tap 54 toan output optical path 56. A small portion, e.g. 5%, of the opticalsignal which is tapped by the optical tap 54 is detected by an opticaldetector 58, whose electrical output is amplified and filtered by an AGCamplifier and filter unit 60, an output of which is supplied to adigital signal processor (DSP) unit 62. The DSP unit 62 provides acontrolled voltage bias to the EVOA 52 in accordance with a respectiveCID for the optical signal, with which the DSP unit 62 is supplied forexample from the NMS 14. The modulator of FIG. 3 can modulate theoptical signal with a desired amplitude modulation depth, for example ofabout 1% to about 4%, at frequencies of for example up to about 1 MHz,to provide the desired form of CID modulation on the optical signal.

[0041] It is observed that the optical signal is also modulated in knownmanner with a data signal typically at a high bit rate. As describedabove with reference to FIG. 2, the CID modulation is applied to eachoptical signal after its data modulation. It can be appreciated thatthis need not be the case, and instead the CID modulation can be appliedto an optical carrier which is subsequently modulated with data to becarried.

[0042]FIG. 4 illustrates one form of detector which can be used in theapparatus of FIG. 2, for implementing each of the CID detectors 32 and40 as shown in FIG. 2.

[0043] Referring to FIG. 4, an optical signal at the multiplexed port ofan optical band filter, for example at the output of the OBF 28 actingas a multiplexer in FIG. 2 or at the input of the OBF 42 acting as ademultiplexer in FIG. 2, on an optical path 64 is supplied to an opticalpath 66 via an optical tap 68. A small portion, e.g. 5%, of the opticalsignal which is tapped by the optical tap 68 is detected by an opticaldetector 70, whose electrical output is amplified and filtered by an AGCamplifier and filter unit 72, an output of which is supplied to a DSPunit 74. The DSP unit 74 derives the CIDs of all of the optical channelswhich are present in the optical signal on the path 64, and providesthese as an output to the NMS 14.

[0044] The forms and functions of the DSP units 62 and 74 depend on theparticular form of CIDs used. For example, the DSP unit 62 mayincorporate a digital synthesizer and the DSP unit 74 may incorporate aFast Fourier Transform (FIT) or Discrete Fourier Transform (DFT)function for detecting CID tones.

[0045] As indicated above, the provision of a respective CID for eachoptical channel, and the detection of the CIDs by the CID detectors, isused in a method of determining the topology of the network by the NMS14 in accordance with an embodiment of this invention. FIG. 5 is a flowchart providing an outline of steps carried out in performing thismethod.

[0046] Referring to FIG. 5, a block 80 represents a first step ofproviding each optical channel with a respective CID, this being carriedout for example as described above with reference to FIGS. 2 and 3. Asubsequent block 82 represents a step of detecting the CIDs andsupplying these to the NMS 14, this being carried out for example at themultiplexed ports of the optical band filters as described above withreference to FIGS. 2 and 4. In this step the NMS 14 is also suppliedwith related information as to the “colour” of, i.e. the wavelengthsadded or dropped by, each respective optical band filter and thedirection of the optical signals, i.e. whether the respective opticalband filter is acting as a multiplexer like the OBF 28 or as ademultiplexer like the OBF 42, as described above with reference to FIG.2.

[0047] A block 84 in FIG. 5 represents a further step of processing,individually for each node in the optical network, the above informationfor the respective node to determine from the detected CIDS inter-nodeoptical paths which lead to or from another node, i.e. for each node,each optical path like the path 34 in FIG. 2 which supplies its opticalsignal to another node, and each optical path like the path 38 in FIG. 2via which an optical signal is supplied from another node. This step canalso determine the topology of optical devices within the individualnodes. A further block 86 in FIG. 5 represents a final step ofprocessing, for the optical network as a whole, the inter-node opticalpaths to determine the overall topology of the optical network. Thesteps represented by the blocks 84 and 86 can also provide warnings oralarms to a network operator in the event that errors or faults aredetermined from the discovered topology.

[0048] Accordingly, in the step 82 the NMS 14 is supplied, for each node10, with the CIDs detected by the CID detectors, such as the detectors32 and 40 as described above, at the multiplex port of each OBF of therespective node, together with information as to the colour of the OBFand the optical signal direction, i.e. whether this multiplex port is anoptical input or an optical output. For each node, the NMS carries outthe step 84 in FIG. 5, for example in accordance with the more detailedsteps of FIG. 6. Thus steps 88 to 100 of FIG. 6 serve to implement thestep 84 of FIG. 5.

[0049] Referring to FIG. 6, in the step 88 the NMS 14 creates, for eachOBF of the respective node 10, three ordered lists of CIDS, referred toas lists M, B, and T. The list M is a list of all of the CIDs detectedby the respective CID detector at the multiplex port of the OBF,supplied in the step 82. The list B is a subset of the list M, limitedto the CIDs in the list M for optical channels with wavelengths that arewithin the colour of the OBF, i.e. these wavelengths are selected by theband filter to be coupled between the add port AD or drop port DR andthe multiplex port of the OBF, so that the list B contains the CIDS ofoptical channels that are added or dropped by the OBF. The list Tcontains the other CIDs, i.e. the CIDs of the list M which are not inthe list B, representing optical channels which are present at thethrough port TH of the OBF. Each of these lists also has an associatedindication of whether it relates to optical channels at an input to oran output from the respective OBF.

[0050] In determining the intra-node and inter-node topology of theoptical network, each of the created lists is used as an identity forthe optical path or fiber at the respective port of the respective OBFand node. Any of these lists created in the step 88 can be a null list,i.e. an empty list containing no CIDS; for example a null list would becreated for an OBF port which is unconnected and carries no opticalsignals. At the step 90 in FIG. 6, null lists are excluded from thefurther topology-determining steps.

[0051] In the step 92, any lists for OBPs for which the lists M and Tare the same (except for the signal input/output association) aretemporarily set aside. Such lists correspond to OBFs for which nooptical channels are added or dropped, so that the lists of CIDs at themultiplex and through ports of the OBF are the same. An example of thisis further described below with reference to FIG. 7.

[0052] In the step 94, optical paths within the respective node 10, i.e.intra-node paths, are determined by the NMS 14 by matching pairs of theremaining lists M and T for the node, one list of each pair being anoutput from one OBF and the other list of the respective pair being aninput to another OBF of the node. Thus each such matched pair of listsalso represents an optical path or fiber between the ports associatedwith the lists.

[0053] In the subsequent step 96, it is concluded that the remaininglists M and T, i.e. those that are not null, set aside, or paired withinthe node, represent ports of the node that are coupled to other nodes,and hence the optical paths or fibers coupled to these ports. The listsB identify their respective ports as add or drop ports, depending uponwhether the ports are input or output ports. Except for OBFs whose listshave been temporarily set aside, the topology of the node, i.e. theoptical paths between OBFs of the node, is thereby determined.

[0054] In the step 98, the list M (or the list T which is the same) ofeach OBP whose lists have been temporarily set aside are matched withthe lists of the optical paths of the node determined in the steps 94and 96, and the determined intra-node topology is expanded to includethese OBFs in the respective optical paths. As further described below,where an optical path includes only one such OBF, its position andoptical connections are thereby fully determined, but for an opticalpath including two or more such OBFs the order of such OBFs within thisoptical path is undetermined (but this is relatively unimportant becausesuch OBFs do not add or drop any optical channels).

[0055] In the final step 100 of FIG. 6, it is concluded that anyremaining matching lists M and T which were set aside in the step 92relate to OBFs which are optically transparent within the node, i.e.these lists relate to at least one OBF which is coupled between ports ofthe node that are coupled to other nodes, but which OBF does not drop oradd any optical channels so that the node is optically transparent tothe associated optical signal.

[0056] For example, if for a particular OBF in the node the list Mrelates to an input port of the OBF and the list T relates to an outputport of the OBF, and these lists are the same and not null, then theyare set aside at the step 92. If at the step 98 these lists are notmatched with another list in the node, then it is concluded at the step100 that the list M for this OBF constitutes an input optical path ofthe node, the list T represents an output optical path of the node, andthat optical signals are coupled transparently, i.e. without anydropping or adding of optical channels, from this node input to thisnode output via this OBF, As in the case of the step 98, if at the step100 it is determined that the matching lists M and T of a plurality ofOBFs in the node are the same, then it is concluded that these OBFs arecoupled in sequence with one another and that the particular order ofthe OBFs in this sequence can not be determined.

[0057] The steps of FIG. 6 are further explained by way of example withreference to FIG. 7, which illustrates one of numerous possiblearrangements of OBFs within a node. As shown in FIG. 7, the node isassumed to contain seven OBFs which are identified as band filters F1 toF7. As illustrated, the OBFs F1 to F4 are arranged as drop ordemultiplexing filters, and the OBFs F5 to F7 are arranged as add ormultiplexing filters. For each of the filters F1 to F7, a rectangle atthe multiplex port of the filter represents the presence of a respectiveCID detector as described above. For the drop filters F1 to F4 themultiplex port is an optical input, and for the add filters F5 to F7 themultiplex port is an optical output, of the respective OBF.

[0058] In the step 88 of FIG. 6, the three ordered lists M, B, and T arecreated by the NMS 14 for each of the OBFs F1 to F7. For clarity, theselists are represented as M1, B1, and T1 for the OBF F1, M2, B2, and T2for the OBF F2, and so on, and these list designations are illustratedin FIG. 7 adjacent to the OBF ports to which they relate. It can beappreciated that the lists M1 to M4, BS to B7, and Ts to T7 areidentified as optical inputs for their respective OBFs, and the otherlists are identified as optical outputs for their respective OBFs.

[0059] As illustrated in FIG. 7, it is assumed that no optical channelsare dropped by the OBFs F2 and F3, and that no optical channels areadded by the OBPs F5 and F7, so that their drop/add ports areunconnected and the lists B2, B3, B5, and B7 are null lists and areexcluded at step 90 in FIG. 6. Further, for each of these OBFs F2, F3,F5, and F7 the respective list T is the same as the respective list M,so that these lists for these OBFs are set aside at step 92 in FIG. 6.

[0060] Of the remaining lists M and T, in the step 94 of FIG. 6 the NMS14 pairs the list T4 (output) with the list M4 (input), and pairs thelist T4 (output) with the list T6 (input), in each case concluding thatthere is an optical path between the ports corresponding to the pairedlists. In the step 96 of FIG. 6 the NMS 14 concludes correctly that theremaining lists M and T, i.e. the lists M1 and M6, correspond tointer-node optical paths, i.e. respectively an input port and an outputport of the node, to which inter-node optical paths or fibers arecoupled. The NMS 14 also recognizes from their input and outputassociations that the lists B1 and B4 relate to optical channels droppedin the node and that the list B6 relates to optical channels added inthe node.

[0061] In the step 98 of FIG. 6, the NMS 14 matches the set-aside listsM2 and M3 each to the list T1 or M4 to determine that the OBFs F2 and F3are present in the optical path T1-M4 as determined above, but it cannot determine the order of the OBFs F2 and F3 within this optical path(i.e. whether the OBF F2 precedes the OBF F3 as illustrated, or whetherthe OBF F3 precedes the OBF F2). In addition, the NMS 14 matches theset-aside list MS to the list T4 or T6 to determine that the OBF F5 ispresent in the optical path T4-T6 as determined above; in this case itconcludes by matching inputs and outputs that there are optical pathsT4-T5 and M5-T6.

[0062] There is no match in the step 98 for the list M7, Accordingly, inthe step 100 of FIG. 6 the NMS 14 concludes that the list T7 (input) andthe list M7 (output) relate respectively to an input port and an outputport of the node, corresponding to inter-node optical paths, and thatthe OBF F7 provides an optically transparent coupling from this inputport to this output port of the node.

[0063] It should be appreciated that the simple example of FIG. 7 isprovided only by way of additional explanation of the steps of FIG. 6,and that these steps can be carried out to determine arbitrarytopologies of nodes.

[0064] In the step 86 of FIG. 6, the NMS performs a sequence of steps todetermine the inter-node, or network, topology in a manner which isgenerally similar to that described above for determining the intra-nodetopology for each node. Steps 102, 104, and 106, illustrated in FIG. 8and further described below, constitute the step 86 of FIG. 6 andgenerally correspond for the network topology to the steps 92, 94, and98 respectively of FIG. 6 as described above for the node topology. Tothis end, the NMS 14 makes use of the lists M and T, e.g. M1, M6, M7,and T7 as described above with reference to FIG. 7, which it hasconcluded in the steps 96 and 100 represent inter-node opticalconnections for each node. A further step 108 in FIG. 8 provides anoptional additional sanity check for the network.

[0065] Referring to FIG. 8, the NMS 14 initially determines at the step92 whether any node has two matching lists for input and output; if sothese are set aside with the conclusion that the node is opticallytransparent to the optical channels to which these lists relate. Usingthe example of FIG. 7, the NMS 14 determines that the respective nodehas matching lists T7 and M7 for two of its inter-node ports, andaccordingly these are set aside in the step 92, the node being opticallytransparent for optical traffic between these ports.

[0066] In the step 104, the NMS 14 determines inter-node paths bymatching pairs of the remaining inter-node lists for the network, onelist of each pair being an output from one node and the other list ofthe respective pair being an input to another node of the network. Anyunpaired lists are concluded to represent errors (e.g. an unconnectedoptical fiber) for which a warning or alarm is provided by the NMS 14 toan operator.

[0067] In the step 106, the matching lists, for input and outputinter-node optical channels, of any node concluded at the step 102 asbeing optically transparent for this traffic, are matched with lists ofthe determined inter-node optical paths. The NMS 14 accordingly expandsthe determined internode topology to include each optically transparentnode in the respective inter-node optical path, in a similar manner tothat described above for intra-node OBFs which do not add or drop anyoptical channels. As in that case, in the event that the same inter-nodeoptical path includes a plurality of nodes which are opticallytransparent to the traffic on the optical path, then the order orsequence of these nodes in the optical path is not determined by the NMS14.

[0068] As indicated by the step 108, the NMS 14 can optionally perform afurther sanity check on the determined topology of the network, thatincoming and outgoing optical paths are symmetrical with respect to theOBFs to which these opposite-direction optical paths are connected, asis normally the case in an optical network. Such a check facilitatesdetecting an error of one node receiving optical signals from anothernode but transmitting optical signals to a third node. Other furtherand/or additional checks may also be performed by the NMS 14; forexample, the determined topology may be compared with a previoustopology of the same network to identify any differences.

[0069] As described above, the CID detectors are provided at themultiplexed ports of the optical filters, but this need not be the caseand the CID detectors can be provided additionally or instead at otherpoints in the optical paths. In addition, as described above there is aninitial process (the steps of FIG. 6) of determining intra-node topologyand hence ports of each node, and a subsequent process (the steps ofFIG. 8) of determining the inter-node network topology, but again thisneed not be the case.

[0070] For example, instead a CID detector may be provided for eachoptical path entering or leaving each node, i.e. for each node port, toprovide a respective list of channels at each node port. The steps 102,104, and 106 of FIG. 8 as described above can be carried out in respectof these lists, to determine the inter-node network topology without anydetermination of the topology within each node.

[0071] Thus although particular embodiments of the invention aredescribed above, it can be appreciated that numerous modifications,variations, and adaptations may be made without departing from the scopeof the invention as defined in the claims.

1. A method of determining topology of an optical WDM (wavelengthdivision multiplex) network in which optical signals comprising aplurality of WDM optical channels are communicated, comprising the stepsof: modulating each optical channel with a respective signal comprisinga channel identity; detecting the channel identities of all of theoptical channels in an optical signal at each of a plurality of pointsin the network to produce a channel list for each of said points; andidentifying matched pairs of channel lists to determine optical paths ofthe network between pairs of said points.
 2. A method as claimed inclaim 1 wherein the step of detecting channel identities comprisesdetecting the channel identities of all of the optical channels in anoptical signal at each of a plurality of optical paths entering orleaving each of a plurality of nodes of the network.
 3. A method asclaimed in claim 1 wherein the step of detecting channel identitiescomprises detecting the channel identities of all of the opticalchannels in an optical signal at a multiplex port of each of a pluralityof optical band filters to produce a respective channel list M, anddetermining a channel list T for a through port of the respectiveoptical band filter, the channel list T comprising channels of the listM which are not within a pass band of the filter.
 4. A method as claimedin claim 3 wherein the step of identifying matched pairs of channellists comprises, for each of a plurality of nodes of the network,identifying matched pairs of channel lists among the channel lists M andT for different optical band filters of the node to determine opticalpaths within the node.
 5. A method as claimed in claim 4 wherein thestep of identifying matched pairs of channel lists further comprises,for each of the nodes, identifying any optically transparent opticalband filters for which the channel lists M and T are the same, andidentifying any channel lists, from among said matched pairs of channellists of the node, matching said same channel lists M and T to determineoptical connections of said optically transparent optical band filterswithin the node.
 6. A method as claimed in claim 1 wherein the step ofmodulating each optical channel with a respective signal comprising achannel identity comprises variably attenuating an optical signal of theoptical channel in dependence upon a signal comprising the respectivechannel identity.
 7. A method as claimed in claim 6 wherein the signalcomprising the respective channel identity has a frequency of the orderof about 1 MHz or less.
 8. A method of determining topology of anoptical WDM (wavelength division multiplex) network in which opticalsignals comprising a plurality of WDM optical channels are communicatedamong a plurality of nodes of the network, comprising the steps of:modulating each optical channel with a respective signal comprising achannel identity; for each of a plurality of optical paths entering orleaving each of a plurality of nodes, determining a channel list of allthe optical channels in an optical signal on the optical path, this stepcomprising detecting the channel identities of all of the opticalchannels in an optical signal at each of a plurality of points; andidentifying matching channel lists to determine optical paths of thenetwork between the nodes.
 9. A method as claimed in claim 8 wherein thestep of determining a channel list of all the optical channels in anoptical signal on an optical path entering or leaving a node comprisesdetecting the channel identities of all of the optical channels in anoptical signal on the respective optical path.
 10. A method as claimedin claim 8 wherein the step of determining a channel list of all theoptical channels in an optical signal on an optical path entering orleaving a node comprises, for each node: detecting the channelidentities of all of the optical channels in an optical signal at amultiplex port of each of a plurality of optical band filters of thenode to produce a respective channel list M; determining a respectivechannel list T for an optical signal at a through port of the respectiveoptical band filter, the channel list T comprising channels of therespective list M which are not within a pass band of the optical bandfilter; identifying matching channel lists M and T to determine opticalpaths within the node; and identifying unmatched channel lists M or T aschannel lists for optical path entering or leaving the node.
 11. Amethod as claimed in claim 10 wherein the step of identifying matchingchannel lists M and T to determine optical paths within the nodecomprises identifying any optically transparent optical band filters ofthe node for each of which the channel lists M and T are the same;identifying matched pairs of the other channel lists M and T for thenode to determine optical paths between respective ports of differentoptical band filters within the node; and identifying any channel lists,from among said matched pairs of channel lists for the node, matchingsaid same channel lists M and T to determine optical connections of saidoptically transparent optical band filters within the node.
 12. A methodas claimed in claim 8 wherein the step of modulating each opticalchannel with a respective signal comprising a channel identity comprisesvariably attenuating an optical signal of the optical channel independence upon a signal comprising the respective channel identity. 13.A method as claimed in claim 12 wherein the signal comprising therespective channel identity has a frequency of the order of about 1 MHzor less.
 14. An optical WDM (wavelength division multiplex) networkcomprising a plurality of nodes and optical paths for communicatingoptical signals, comprising a plurality of WDM optical channels, withinand among the nodes, the network comprising: a source for each opticalchannel; a modulator for modulating each optical channel with arespective signal comprising a channel identity; a plurality of opticalfilters for combining optical channels to produce optical signals andfor separating optical signals to derive optical channels from theoptical signals; a plurality of detectors for detecting the channelidentities of all of the optical channels in an optical signal at eachof a plurality of points in the network to produce a channel list foreach of said points; and a network management system for identifyingmatched pairs of said channel lists to determine optical paths of thenetwork between pairs of said points.
 15. An optical WDM network asclaimed in claim 14 wherein the optical filters comprise optical bandfilters each having a multiplex port, an add or drop port, and a throughport, and said plurality of points in the network comprise multiplexports of the optical band filters.
 16. An optical WDM network as claimedin claim 15 wherein the network management system is arranged todetermine a channel list for a through port of an optical band filter byomitting, from optical channels of a channel list for the multiplex portof the respective optical band filter, optical channels within a passband of the optical band filter.